The recovery of hydrocarbons from subterranean zones relies on the process of drilling wellbores. The process includes drilling equipment situated at surface, and a drill string extending from the surface equipment to a below-surface formation or subterranean zone of interest. The terminal end of the drill string includes a drill bit for drilling (or extending) the wellbore. The process also involves a drilling fluid system, which in most cases uses a drilling fluid (“mud”) that is pumped through the inside of piping of the drill string to cool and lubricate the drill bit. The mud exits the drill string via the drill bit and returns to surface carrying rock cuttings produced by the drilling operation. The mud also helps control bottom hole pressure and prevent hydrocarbon influx from the formation into the wellbore, which can potentially cause a blow out at surface.
Directional drilling is the process of steering a well from vertical to intersect a target endpoint or follow a prescribed path. At the terminal end of the drill string is a bottom-hole-assembly (“BHA”) which comprises 1) the drill bit; 2) a steerable downhole mud motor of a rotary steerable system; 3) sensors of survey equipment used in logging-while-drilling (“LWD”) and/or measurement-while-drilling (“MWD”) to evaluate downhole conditions as drilling progresses; 4) means for telemetering data to surface (“telemetry tool”); and 5) other control equipment such as stabilizers or heavy weight drill collars. The BHA is conveyed into the wellbore by a string of metallic tubulars (i.e. drill pipe). MWD equipment is used to provide downhole sensor and status information to surface while drilling in a near real-time mode. This information is used by a rig crew to make decisions about controlling and steering the well to optimize the drilling speed and trajectory based on numerous factors, including lease boundaries, existing wells, formation properties, and hydrocarbon size and location. The rig crew can make intentional deviations from the planned wellbore path as necessary based on the information gathered from the downhole sensors during the drilling process and transmitted to surface by the telemetry tool. The ability to obtain real-time MWD data allows for a relatively more economical and more efficient drilling operation.
One type of downhole telemetry known as mud pulse telemetry involves creating pressure waves (“pulses”) in the mud circulating through the drill string. Mud is circulated from surface to downhole using positive displacement pumps. The pressure pulses are created by a fluid pressure pulse generator in the downhole telemetry tool, which controllably changes the flow area and/or path of the mud as it passes through the pulse generator in a timed, coded sequence, thereby creating pressure differentials in the mud. A number of encoding schemes can be used to encode data into mud pulses. These schemes include amplitude phase shift keying (ASK), frequency shift keying (FSK), phase shift keying (PSK), or a combination of these techniques. The choice of modulation scheme uses a finite number of distinct signals to represent digital data, known as symbol sets. PSK uses a finite number of phases, each assigned a unique pattern of binary digits. Usually, each phase encodes an equal number of bits. Each pattern of bits forms the symbol that is represented by the particular phase.
The pulses travel to surface and are received and decoded by a surface receiver which reconstructs the data sent by the telemetry tool. The surface receiver comprises a demodulator which is designed specifically for the symbol-set used by the modulator, determines the phase of the received signal and maps it back to the symbol it represents, thus recovering the original data. Undesired background noise in the mud medium between the telemetry tool and the surface receiver can be caused by a number of sources, including a mud pump, surface rig, other telemetry signals, and the surrounding reservoir formation. This undesired background noise can interfere with the reception of the telemetry signal; conventional approaches to dealing with undesired noise include applying one or more signal processing techniques at surface to separate the undesired noise from the subject telemetry signal. However, successfully decoding a pressure pulse telemetry signal transmitted in a particularly noisy environment can be quite challenging using conventional approaches, and a new solution is desired to meet these challenges.